The invention relates to an arrangement for the glare-free illumination of instruments or, rather the indicator markings on the face of an instrument, which arrangement requires no external light source. The arrangement is especially suitable for instruments with movable indicators, that is, with hands or movable displays which may be provided with luminescent or phosphorescent substances.
It is often necessary for instruments to be capable of being read accurately without external illumination. This is true, for example, for instruments of cars and those in other land vehicles, ships and airplanes, where, at night, external illumination would disturb the driver or pilot since the adaption of the eye to the external light would impair the driver's or pilot's night vision. To avoid such impairment, the instruments are usually lighted by one or more small light sources whose energizing level is subjectively adjustable by the driver depending on the adaptability of his eyes. Generally, these light sources provide visible light. A disadvantage of this kind of instrument illumination is that the light sources may cause glares and that the adaptability of the eyes to the surrounding light conditions is noticeably reduced since the eyes adjust to the lightest area within the field of vision, and with this kind of instrument illumination, the lightest areas are the lighted instrument panels. Glares by reflections on the instrument panels and glasses may be a further aggravation. Since this kind of lighting is generally provided by incandescent lamps, there is also a substantial consumption of electrical energy which is particularly aggravating during a loss of regular power when an emergency power supply must be utilized since illumination of the necessary instruments must continue. Taking the possibility of loss of power into consideration, it has been made imperative for the operators of passenger ships and airplanes to always carry an external movable light source (a flashlight, for example). The high energy consumption of the lighting equipment also requires emergency supplies of high capacity and the large currents generate relatively large electromagnetic fields which sometimes cause disturbance of the highly sensitive instruments.
In order to be visible at least during dawn at the end of daylight and also for a limited time early in the night without annoying illumination, displays have been provided with phosphorescent materials for some time already.
The phosphorescent materials have atoms which, when excited by light energy, retain part of the energy and release it only slowly within minutes or within hours depending on the phosphorescent material. The length of the light emission period of the phosphorescent material depends also on the preceding excitation time or rather the excitation energy absorbed and, furthermore, on the temperature of the materials during light emission. Intensity and length of time of light emission is, therefore, variable so that it is not possible to rely solely on such phosphorescent materials as means for illuminating vital instruments.
It is, therefore, the principal object of the present invention to provide an arrangement for illuminating instruments which, in addition to requiring only a relatively small amount of electrical energy, does not impair adaption of the eyes to a low level of light of the surroundings but which, nevertheless, will easily permit recognition of instrument display characters.